1. Field of the Invention
This invention relates to improved positive electrodes for use in rechargeable cells, and a process for producing same. More particularly the invention pertains to positive electrodes wherein a paste containing an electrochemically active material and a binder is coated on at least a portion of at least one of the opposing major surfaces of a two-dimensional electrically conductive substrate.
2. Description of Prior Art
There are a number of processes to form positive electrodes for use in rechargeable cells. One well known method involves sintering. Sintered positive nickel hydroxide electrodes are typically formed by sintering nickel powder at elevated temperatures to form a porous three dimensional plaque and then impregnating this porous structure with nickel salts which are subsequently converted to nickel hydroxide. As used herein, a three-dimensional substrate refers to a porous substrate having at least a portion of its interior surface area in contact with electrochemically active material. U.S. Pat. No. 2,724,733, for example, describes a sintered electrode structure for a nickel cadmium cell having nickel hydroxide as the active material for the sintered positive electrode. U.S. Pat. No. 3,826,684 also describes a nickel cadmium cell having a sintered positive nickel electrode containing additives to enhance charge acceptance. However, the manufacturing process for the sintered electrode is capital intensive. The porous three-dimensional sinter plaque structure is impregnated by immersing the plaque in a nickel salt bath and converting the nickel salt to nickel hydroxide by the reaction of the salt with caustic. In order to obtain an electrode containing a sufficient amount of active material, this immersion step must be repeated several times. The sintered nickel electrode therefore requires a costly, complicated and time consuming process for its fabrication.
Another process to form the positive electrode involves forming a paste from nickel hydroxide and other additives and depositing the paste into an appropriate three-dimensional substrate. Such three-dimensional substrates are typically formed by nickel plating polyurethane foams, non woven felts and carbon fiber mats, and are referred to in the art as sponge metal, foamed porous metal, porous metal matrix, and felt. U.S. Pat. Nos. 4,251,603 and 4,582,098 are representative of processes which utilize a sponge-like three-dimensional porous metal matrix to form the electrode. Specifically, U.S. Pat. No. 4,251,603 describes a battery electrode having a carrier made of a sponge-like porous metal matrix having a multiplicity of spaces throughout. An active material paste fills the matrix followed by drying, calendaring and cutting to the desired size. U.S. Pat. No. 4,582,098 describes a method of spraying a pasty mixture into a porous three-dimensional metal body from both sides so as to fill the pores with the mixture. As compared to sintered electrodes, foam electrodes provide greater capacity, e.g., 1250 mAH as compared to 1100 mAH in a typical AA cell. The higher porosity of the foam material is believed to contribute to the greater capacity of the foam electrode. However, the materials and manufacturing costs for foam positive electrodes are also high.
Both of these processes for manufacturing a positive electrode require that a porous three-dimensional substrate be filled with active material, a process more costly and complex than a coating process. Efforts have been undertaken to provide for a rechargeable positive electrode with a two dimensional current collector substrate supporting a layer of active material on each major surface of the substrate. As used herein a two-dimensional current collector substrate refers to a substrate such as a foil sheet where the substrate thickness is not coextensive with the electrode thickness and the electrochemically active material contacts essentially the external surface area of the substrate. Volume changes that occur in the positive electrode have hampered efforts to provide satisfactory adhesion between the two dimensional conductive substrate and the active material when the material is coated onto the substrate. As a result, the active material separates from the substrate, reducing capacity and increasing resistance. Various non-coating methods for forming a positive electrode with a two dimensional current collector substrate have been proposed. For example, U.S. Pat. No. 3,898,099 describes an electrode sheet formed of active material and PTFE fibers as a binder wherein the sheet is subsequently pressed onto a metal screen or foil current collector. This process is complex in that the active material electrode sheet is formed by repeated working of the paste by rolling and folding in order to form a cohesive sheet. A current collector substrate is then sandwiched between sheets to form the electrode assembly followed by additional pressure to the assembly to form the finished electrode. Another proposed solution to the adhesion problem for a positive electrode using a two-dimensional substrate is to corrugate a two dimensional substrate and then apply a thin sintered metal layer on the surfaces of the corrugated substrate prior to coating with an active material. The required additional working of the substrate to provide a corrugated profile adds additional manufacturing costs to the electrode.
Accordingly, there is a need in the art for a positive electrode that can be manufactured using a simple coating process, and that is cost competitive with the sintered positive electrode and performance competitive with the foam positive electrode.
It is therefore one object of the present invention to provide an improved positive nickel electrode for rechargeable electrochemical cells having comparable or enhanced capacity as compared with sintered and foam positive electrodes.
Yet another object of the present invention is to provide a simple method of manufacturing a positive nickel electrode for a rechargeable cell by coating a two dimensional substrate with a paste comprising active material and a binder.
To achieve the foregoing and other objects and advantages and in accordance with a purpose of the present invention, as embodied and broadly described herein, a positive electrode is provided comprising a two-dimensional electrically conductive substrate having at least a first major surface and a second major surface opposing said first surface, and a coating contacting at least a portion of at least one of said first and second surfaces, said coating comprising nickel hydroxide as an active material in the discharged state and a binder, wherein the porosity of the final electrode coating is from about 15 to 35 percent. Preferably, said substrate thickness is between 10 and 50 percent of the final electrode thickness. Said binder is preferably a styrene-ethylene/butylene-styrene triblock copolymer binder. Preferably said nickel hydroxide active material is co-precipitated with zinc and cobalt. Other metal hydroxides as are known in the art can also be used. Preferably, said substrate comprises perforated nickel plated steel strip with nickel powder sintered to at least a portion of said strip. Said coating can additionally comprise additives of nickel, nickel hydroxide, cobalt, cobalt oxide, cobalt hydroxide, carbon, graphite, zinc oxide or zinc hydroxide, or combinations thereof.
The within invention additionally provides a method for making such an electrode, comprising the steps of preparing a paste preferably comprising said active material and said binder in a solvent, delivering said paste to a coating or extrusion device and feeding said substrate through the coating die while the paste is preferably applied to both major surfaces of the substrate. Preferably, the electrode is then passed through an oven to remove the solvent, and calendared to a desired thickness that provides the desired coating porosity.